You should first try remembering the password as best you can, fiddling around in the registry is a last resort. Think of all the usual passwords you might use. Remember that passwords are CaSe sensitive, so 'Munky' is not the same as 'MUNKy' or 'MuNkY'.
If all else fails, you'll have to go through the registry. There are instructions to follow on
this page. Make sure you have the correct key. Deleting a wrong key can potentially cause your computer to stop working. To access RegEdit, it's easier to go to
Start > Run and type it 'regedit', and click 'Ok'.
G'luck

(Write it down somewhere safe in future! *grin*)
Regards,